In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
In the LTE uplink a Sounding Reference Signal (SRS) is a type of reference signals that has been incorporated into an Evolved Universal Terrestrial Radio Access (E-UTRA) standard of a Third Generation Partnership Project (3GPP). The SRS may be used for many different purposes such as estimation of a time-of-arrival and a frequency offset; calculation of beam-forming weights in the downlink; power control in the uplink; and for Frequency Selective Scheduling (FSS) and Frequency Selective Link Adaptation (FSLA) in the uplink.
SRS may be configured to be transmitted in a single transmission over the total system bandwidth or be split into several, more narrow band SRS transmissions e.g. 4 Physical Resource Blocks. Each PRB may comprise 12 subcarriers of 15 kHz equating to 180 kHz of spectrum. The total system bandwidth is then covered through the utilization of frequency hopping, wherein SRSs of subframes are spread in frequency relative one another covering the total system bandwidth of frequencies.
Existing known technologies use SRS standalone as the basis for estimating a channel gain input to FSS and FSLA. Channel gain is measured by measuring received signal power given the transmit power, and depends on pathloss, antenna gain and feeder loss etc. For this to work well, SRS has to be transmitted quite frequently both in the time and frequency domain in order for the scheduling to be able to follow the time and frequency variations of the radio channel. The LTE standard allows the time transmission intervals (TTI) to be configured per UE, which can independently be set to a periodicity of 2, 5, 10, 20, 40, 80, 160 and 320 ms. By using the most frequent SRS transmission setting of 2 ms a very good resolution in the time domain can be achieved. Transmitting SRS this frequently will however consume a lot of SRS resources for a single UE and also introduce a lot of interference on the SRS channel that will impact the accuracy of the SRS. Therefore there is a trade-off between the number of configured SRS UEs that can be supported and the amount of time resolution that can be achieved.
Furthermore existing technologies rely on updating the channel gain estimation vector on those frequencies where a SRS transmission has been received. This is done by overwriting old channel gain measurements with the new channel gain measurements. The drawback of using the overwriting approach is that during frequency hopping schemes it may take a very long time before each frequency is updated. If for example SRS is transmitted every 5th ms and a 4-PRB wide SRS is transmitted it will then take 120 ms between updates for 20 MHz bandwidth covering 96 PRBs. This makes the channel gain estimation quite unreliable even for slowly varying radio channel conditions. If the channel is varying fast enough each new SRS update will appear as noise in the scheduler.
As mentioned earlier SRS can be configured to be transmitted at different bandwidths, and in its simplest form the SRS is transmitted over the full system bandwidth in one single transmission. The drawback with the full bandwidth approach is that the available power in the UE will then have to be distributed over many frequencies or subcarriers. If the SRS transmissions are split into several narrower SRS transmissions, the available power can be distributed over fewer subcarriers, thereby improving the SRS coverage. On a 20 MHz LTE system a 6 dB gain in link coverage is achieved by using 24PRB SRS and frequency hopping compared to using SRS transmissions over the full bandwidth. If a 4-PRB wide SRS is chosen a 13.8 dB gain in link coverage may be achieved compared to a full bandwidth configuration. By utilizing SRS transmissions over narrower band and frequency hopping the link budget for the SRS channel, which link budget defines available resources for the SRS channel may be significantly improved. This is important since FSS and FSLA then may be used as a feature for improving the coverage and thereby also the throughput at the cell edge. Hence, a solution for maximizing the uplink coverage, narrow band SRS together with frequency hopping may be employed. However, as mentioned above, problems when using SRS transmissions of narrower band and frequency hopping are that repeatedly sent SRSs consumes a lot of radio resources and that updated SRS measurements cannot keep up with varying conditions of the channel.